The present invention relates to seals between turbine bucket covers and casings surrounding the bucket covers and particularly relates to combination brush/labyrinth seals for securement to casings and engaging steam turbine bucket covers.
Brush seals are typically used to minimize leakage through the gap between stationary and rotating components, for example, between high and low pressure regions on opposite sides of the seal. Brush seals are typically formed by disposing a plurality of metal or ceramic bristles between a pair of backing plates typically secured to one component of the seal. The bristles have tips which extend beyond the margins of the backing plates for engagement with the other seal component, e.g., the rotating component. Thus, in a steam turbine where a stationary component receives a shaft, the brush seal is typically secured in a groove formed along an inside diameter of the stationary component, with the backing plates and bristles projecting radially inwardly toward the shaft. The bristle tips project beyond the inside diameter of the backing plates and the bristles form a seal with the rotating shaft.
For original equipment, the brush seal is custom-manufactured for fit with the stationary component. Thus, the backing plates are normally formed as annular rings with the bristles disposed between the rings and secured thereto. The annular ring is, of course, formed to the desired diameter. The ring and brush seals are then cut into brush seal segments for fit into the stationary component. Typically, a number of such segments (e.g., six to twelve for shaft seals, between eight to twenty-six segments for tip seals) are arranged in a stationary component, e.g., in a casing or diaphragm appendage about the tips of buckets in a steam turbine. The manufacturing cycle time, however, is substantial as the brush seals are made for and to a specific diameter and require substantial lead time for manufacturing purposes. Also, the brush seal, when used in conjunction with new packing rings, may have the brush seal backing plate machined integrally with the new packing ring. Where the brush seal, however, is used as a tip spill that is inserted directly into the diaphragm, the backing plate cannot be machined into the existing diaphragm. In all these cases, the brush seal requires custom manufacture to a specific diameter and cannot be used for other and different diameters. Accordingly, there is a need for a combined brush/labyrinth seal which can reduce manufacturing time by avoiding custom-fitting of the brush seal and facilitate retrofit of brush seals in existing casings or diaphragms, for example, in steam turbines for sealing with bucket covers.
In accordance with a preferred embodiment of the present invention, there is provided a brush seal insert formed of a plurality of metal or ceramic bristles disposed between a pair of mounting elements, e.g., flexible bars. Particularly, the bars are flexible in the direction of bending about an axis in which the bars and bristles and, hence, the brush seal, will be situate. The bars and bristles are therefore flexible to various diameters over a wide range of diameters. This is in contrast to conventional brush seals employing backing plates dimensionally thick in the direction of bending which cannot readily be bent to accommodate different diameters of brush seals. Thus, the brush seal portion, i.e., insert, of the combined brush/labyrinth seal can be manufactured ahead of any need for the brush/labyrinth seal combination over a wide range of diameters. This minimizes the manufacturing cycle time when an actual need for a new brush seal arises, e.g., for retrofitting a combination brush/labyrinth seal into a diaphragm.
In a preferred embodiment of the present invention, the combination brush/labyrinth seal is provided in a diaphragm groove. For example, the groove can be formed in an otherwise smooth diaphragm to receive the base portion of a labyrinth tooth. The tooth may have a step which receives one of the bars of the brush seal, while the other bar is received in a recess formed in the diaphragm surface. To secure the combined brush/labyrinth seal in the diaphragm groove, the material of the diaphragm may be peened over the brush/labyrinth seal at circumferentially spaced positions within the groove.
In another embodiment of the present invention, the labyrinth tooth may have a hook fit with the groove in the diaphragm. That is, one or a pair of axial flanges along the base of the labyrinth tooth may project into one or a pair of correspondingly-shaped recess(es) in the groove of the diaphragm. Thus, the combined labyrinth tooth and brush seal may be inserted in a circumferential direction at the midline of the diaphragm, with the edge of the diaphragm being peened along an edge of one of the brush seal bars to retain the labyrinth tooth/brush seal in the diaphragm.
In a preferred embodiment according to the present invention, there is provided in a turbine having a plurality of rotatable buckets with bucket covers and a stationary component surrounding the buckets, a method of forming a seal between the bucket covers and the stationary component, comprising the steps of forming a brush seal having a plurality of bristles extending in a generally common plane and flanked at proximal ends of the bristles by a pair of flexible bars, forming a labyrinth seal tooth segment to a predetermined arc, flexing the brush seal about an axis normal to the common plane to the predetermined arc, forming an annular groove in the stationary component and securing the tooth segment and the brush seal in engagement with one another and in the groove with the bristles engageable with the bucket covers and an edge of the tooth segment closest to the bucket covers being spaced from the covers.
In a further preferred embodiment according to the present invention, there is provided a seal for a turbine, comprising a plurality of rotating buckets including bucket covers and a surrounding stationary component, the stationary component having an annular groove, an arcuate labyrinth seal tooth disposed in the groove, an arcuate brush seal including a plurality of bristles and a pair of arcuate mounting elements on opposite sides of the bristles securing the bristles therebetween along an outer diameter of the brush seal, the brush seal disposed in the groove on the upstream side of the tooth with the tips of the bristles along the inner diameter of the brush seal engaging the bucket covers, the tooth having a step along an upstream face thereof and located in the groove, the step receiving a mounting element of the pair thereof and means for securing the tooth and the brush seal in the groove.